The present invention relates to a liquid crystal cell and method thereof. More particularly, the liquid crystal cell contains a chromonic lyotropic liquid crystal that exhibits homeotropic or hybrid bulk alignment.
A liquid crystal is a state of matter in which molecules exhibit long-range orientational order and wherein long-range positional order is either reduced (two-dimensional positional order in columnar phases and one-dimensional positional order in smectic phases) or absent (nematic phases). Accordingly, liquid crystals fall between crystalline solids (which possess both positional and orientational order) and isotropic fluids (which exhibit no long-range order). Solid crystal or isotropic fluid can be transformed into a liquid crystal by changing temperature (creating a thermotropic liquid crystal) or by using an appropriate diluting solvent to change the concentration of mesomorphic molecules (creating a lyotropic liquid crystal).
Alignment of thermotropic liquid crystals is an active area of research and development. Usually, the alignment technique is based on a special unidirectional treatment of the plates or substrates that bind the liquid crystalline material. Such techniques are disclosed in U.S. Pat. No. 5,596,434 entitled “Self-Assembled Monolayers for Liquid Crystal Alignment.” The '434 patent discloses that the plates are covered with a polymer (such as polyimide) layer which is mechanically rubbed. The direction of rubbing sets the direction of orientation of the thermotropic liquid crystal, i.e., the director, at the substrate, as a result of anisotropic molecular interactions at the interface. The phenomenon of orienting action between the anisotropic (rubbed, for example) substrate and the liquid crystalline alignment are called “anchoring.” Alignment by surface anchoring is a standard means of alignment in thermotropic liquid crystalline displays. Surfaces are typically treated with a polymer or a surfactant in order to obtain the desired alignment effects. The methods of alignment are well established for thermotropic liquid crystals but are not necessarily applicable to lyotropic liquid crystals because of the differences in the molecular structure between the two classes of liquid crystals.
Lyotropic liquid crystals are more difficult to align than their thermotropic counterparts. The reason is that most lyotropic liquid crystals are based on amphiphilic materials (surfactants) dissolved in water or oil. Amphiphilic molecules have a polar (hydrophilic) head and a non-polar (hydrophobic) aliphatic tail. When surfactant molecules are in contact with a substrate, their amphiphilic nature generally results in a perpendicular orientation of the molecule with respect to the plane of the substrate. Either the polar head or the hydrophobic tail of the molecule is attracted to the bounding plate, which results in the perpendicular alignment of the molecule with respect to the substrate. Perpendicular alignment means that the preferred orientation is the so-called homeotropic alignment, in which the optical axis is perpendicular to binding plates.
An exemplary lyotropic liquid crystal cell is designated generally by the numeral 10 in FIG. 1. The cell 10 includes a pair of opposed substrates 12, which are sealed in a well-known manner, that contain surfactant-based lyotropic liquid crystal material designated generally by the numeral 14. The material 14 is formed using water 16 as a solvent for biphilic molecules 18. Each of the liquid crystal molecules 18 possesses polar (hydrophilic) parts 20 and apolar (hydrophobic) parts 22. When water 16 is added to biphilic molecules 18, such as the cationic surfactant cetylpiridinium chloride [C21H38ClN], a bilayer 26 forms as the hydrophobic regions coalesce to minimize interaction with the water 16 while enhancing the polar component's interaction with water. The concentration and geometry of the specific molecule define the supramolecular order of the liquid crystal. The molecules can aggregate into lamellae as well as disk-like or rod-like micelles, or, generally, aggregates of anisometric shape. Lyotropic liquid crystals are usually visualized as ordered phases formed by the rod-like surfactant molecules 18 (such as C21H38ClN molecules) in water. These anisometric aggregates form a nematic, smectic, columnar phase, of either non-chiral or chiral (cholesteric phase) nature. For example, the C21H38ClN molecules form a stack of lamellae of alternating layers of water and biphilic molecules, thus giving rise to a lamellar smectic A phase. The molecules on average are oriented along the direction schematically shown by a thick vertical arrow 28 called the director n. On average, the surfactant molecules are oriented along the director n. Surfactant molecules and thus the director n orient normally perpendicular to the bounding plates 12 (so-called homeotropic orientation).
There is a special class of lyotropic liquid crystals, called lyotropic chromonic liquid crystals (LCLC). The LCLC family embraces a range of dyes, drugs, nucleic acids, antibiotics, carcinogens, and anti-cancer agents. The molecular and macrostructure of LCLC's are markedly different from that of conventional lyotropic liquid crystals based on amphiphilic rod-like molecules with polar heads and hydrophobic alkyl chain tails, also referred to as surfactants. LCLC molecules are believed to be plank-like rather than rod-like, rigid rather than flexible, aromatic rather than aliphatic. The π-π interaction of the aromatic cores is the main mechanism of molecular face-to-face stacking according to Lydon [J. Lydon, Chromonics, in: Handbook of Liquid Crystals (Wiley-VCH, Weinheim, 1998) v. 2B, p. 981 and Current Opin. Col. Inter. Sci. 3, 458 (1998)]. Hydrophilic ionic groups at the periphery of the molecules make the material water-soluble. These materials have become a subject of intensive studies lately as it became clear that they can be used as internal polarizing elements in liquid crystal displays, see T Sergan et al., Liquid Crystals v. 5, pp. 567-572 (2000) and in the amplification and detection of ligands as disclosed in U.S. Pat. No. 6,171,802. These applications are enhanced by a uniform alignment of LCLC materials with the director in the plane (planar) of the cell (or slightly tilted).
It has been disclosed by Ichimura et al that inclusion of a non-ionic surfactant into an LCLC such as disodium chromoglycate results in a material that can be aligned by a photoirradiated polymer which incorporates azobenzene groups. But, inclusion of the surfactant material introduces undesirable impurities which can adversely affect the interaction between the liquid crystal and the ligands. Moreover, only photosensitive polymers are shown to orient the liquid crystal material without conclusive reasoning as to how this is achieved.
Reliable techniques for planar alignment of surfactant-free LCLC materials by means of surface anchoring are known and described in U.S. Pat. No. 6,411,354. To align the LCLC in a planar fashion a polymer alignment layer is disposed on the substrate and either rubbed or otherwise physically modified to impart a uniform orientation to the liquid crystal material. The alignment material is a polymer that yields a homeotropic (perpendicular) alignment of standard thermotropic liquid crystals. When the homeotropic polymer surface is rubbed slightly, the nematic phase of the Cromolyn LCLC aligns in the direction of rubbing. A similar surface rubbing gave a high pretilt angle for a typical thermotropic liquid crystal, 5CB. This fact demonstrates that one is able to align LCLCs with surface treatments that exhibit high pretilt angles for thermotropic liquid crystals. Other alignment techniques are also believed to be viable with LCLC material. For example, angular deposition of SiOx could be used as an alignment layer. Alternatively, irradiation of a photosensitive polymer with polarized light could be used. Of course, only photosensitive polymers can be used in this technique which may adversely alter the properties of the liquid crystal for its intended use in this application.
Advantageously, the present invention provides a new type of liquid crystal cell that contains a chromonic lyotropic liquid crystal exhibiting homeotropic or hybrid bulk alignment. The invention may be widely used in, for example, biosensing, detection and amplification of ligands, fabrications of optical elements such as optical retarders including compensators or highly effective optical compensating films or layers, and photovoltaics, among other applications.